The present invention relates, in general, to the rejection of the image signal of a radio frequency signal converted to an intermediate frequency and, in particular, to the tuning or calibration of a frequency conversion mixer especially suited to be implemented on an integrated circuit.
Unlike wireline communications, the wireless environment accommodates essentially an unlimited number of users sharing different parts of the frequency spectrum and very strong signals coexist next to very weak signals. A radio receiver must be able to select the signal of interest, while rejecting all others.
Among the important problems faced by the designers of radio receivers are image rejection and monolithic integration. A radio frequency receiver must be able to select the desired signal from its image. Otherwise, the subsequent detector circuit will be unable to distinguish between the desired signal and the image signal and, therefore, the output will be the result of the superposition of both. As wireless communications units evolve, means to reduce cost, size, and weight through monolithic integration are critical.
Image rejection relates to the ability of the radio frequency receiver to select the desired signal from the image of the desired signal spaced away by twice the intermediate frequency signal. This is important as the subsequent detector circuit will be unable to distinguish between the desired and image signals and, therefore, the output of the detector circuit will be a result of the superposition of both. This is the essence of the image signal problem.
In modern radio frequency receivers for wireless applications, when the problem of canceling unwanted image frequency response is handled in the mixer, typically 50 dB of image filtering is required from the overall system. This image filtering comes from a combination of pre-select band pass filtering, image filtering and possible use of an image reject filter. The high image rejection requirement means that the contribution of each circuit to the overall image rejection is critical.
FIG. 1 shows a conventional image reject mixer. An input radio frequency (RFin) is supplied to two mixers 10 and 12. A first local oscillator signal (LO1) also is supplied to mixer 10 and a second local oscillator signal (LO2), in phase quadrature with the first local oscillator signal, is supplied to mixer 12. Mixer 10, responsive to the radio frequency input signal the first local oscillator signal, develops a first intermediate frequency signal and mixer 12, responsive to the radio frequency input signal the second local oscillator signal, develops a second intermediate frequency signal. The intermediate frequency signals pass through low pass filters 14 and 16 and the first intermediate frequency signal is shifted in phase by 90xc2x0 by a phase shifter 18. The phase shifted first intermediate frequency signal and the second intermediate frequency signal are combined in an intermediate frequency combiner 20 which develops the output intermediate frequency output signal (IFout).
FIG. 2 shows a conventional intermediate frequency combiner that can be used in the FIG. 1 image reject mixer. This intermediate frequency combiner has two differential amplifiers 22 and 24. For the intermediate frequency combiner to function properly, differential amplifiers 22 and 24 should be in phase quadrature and should have equal gain. Differential amplifiers 22 and 24 are in phase quadrature when the pole frequency of feedback capacitor 26 in differential amplifier 22 is ten times lower than the intermediate frequency and the differential amplifiers have equal gain when the magnitude of the capacitance reactance in differential amplifier 22 is equal to the resistance of feedback resistor 28 in differential amplifier 24.
Problems arise when there is degradation in image rejection in an image reject mixer due to process variations (i.e., variations in the values of components, such as feedback resistor 28 in differential amplifier 24 or feedback capacitor 26 in differential amplifier 24 in the intermediate frequency combiner).
To overcome the shortcomings of the prior ways of achieving image signal rejection considered above, a new image reject mixer is provided by the present invention. One object of the present invention is to provide a new and improved image reject mixer. Another object of the present invention is to provide a new and improved radio frequency receiver. A further object of the present invention is to provide a new and improved image reject mixer that is particularly suited for implementation in an integrated circuit.
Accordingly, an image reject mixer, constructed in accordance with the present invention, includes means for supplying a radio frequency input signal and means for supplying a first local oscillator signal and a second local oscillator signal in phase quadrature with the first local oscillator signal. Also included in this image reject mixer are a first mixer responsive to the radio frequency input signal and the first local oscillator signal for developing a first intermediate frequency signal and a second mixer responsive to the radio frequency input signal and the second local oscillator signal for developing a second intermediate frequency signal. An image reject mixer, constructed in accordance with the present invention, further includes means for phase shifting the first intermediate frequency signal by 90xc2x0 and an intermediate frequency combiner for developing an intermediate frequency output signal. The intermediate frequency combiner has a first differential amplifier responsive to one of the second intermediate frequency signal and the 90xc2x0 phase shifted first intermediate frequency signal and having a tunable reactance feedback path and a second differential amplifier responsive to the other of the second intermediate frequency signal and the 90xc2x0 phase shifted first intermediate frequency signal and having a resistance feedback path. Also included in this image reject mixer are means for tuning the reactance feedback path of the first differential amplifier to place the first differential amplifier and the second differential amplifier in phase quadrature when the pole frequency of the reactance feedback path is at least ten times lower than the frequency of the intermediate frequency and to set the gain of the first differential amplifier equal to the gain of the second differential amplifier when the reactance of the reactance feed back path in the first differential amplifier is equal to the resistance of the resistance feedback path in the second differential amplifier.
It is to be understood that the foregoing general description of the invention and the following detailed description of the invention are exemplary, but are not restrictive of the invention.